1. Field of the Invention
The present invention relates to an ink jet recording apparatus and method for recording images, for example, by ejecting ink on a recording medium.
2. Description of the Prior Art/Related Art
In a serial-type recording apparatus, in which images are recorded on a recording medium by scanning the recording head in the main-scan direction and, moving the recording medium in the sub-scan direction defined to be vertical to the main-scan direction, after setting the recording medium at a designated recording position, images are recorded with the recording head mounted on the carriage moving in the width direction on the recording medium, which is designated "main scanning", and every time after completing the recording operation for the single line in the width direction, the recording medium is fed in a designated length in the direction vertical to the above mentioned width direction, which is designated "pitch feed", and thus, by repeating the main scanning operation and the pitch feed operation, the recording of the image on the sheet of the recording medium is performed.
Among the recording apparatus using the serial type recording operation mentioned above, an ink jet recording apparatus using ink jet method records images by ejecting ink on the surface of the recording medium from the recording head, which enables the reduction of the size of the recording head and can establish highly precise recording images with high speed.
In addition, the ink jet recording apparatus has several advantageous aspects such as lower running cost due to the capability of recording on general purpose paper sheets without special chemical surface processing and as lower noise level due to using non-impact recording mechanism, and as the capability of printing colored images with multiple color ink.
Specifically, the recording head used with ink jet method in which ink droplets are ejected by thermal energy can be fabricated by semiconductor fabrication processing such as etching, vacuum evaporation and spattering which are used for forming electro-thermal conversion devices, electrodes, fluid walls and upper plates, each defined on the semiconductor substrate, and ultimately, in the recording head fabricated in this manner, fine-pitched and highly-integrated liquid paths and orifices can be easily formed, and thus, the size of the recording head can be further reduced.
In the above described ink jet recording apparatus, fine-pitched orifices are arranged. In case that air voids and dusts come into the inside the orifice, or that the recording head can not be used for normal recording operations due to the increase of viscosity of ink as the solvent of ink evaporates, ejection recovering operations are performed by flushing off ink staying inside the orifice in order to remove the above described factors for making ink fail to be ejected.
In such an ink jet recording apparatus as described above, the recording medium is wound at a feed roller, and fed by feed rollers and an extraction roller. FIG. 1 shows the cross sectional view of the ink jet recording apparatus, in which what is shown is the feeding state of the recording medium 1. The recording medium 1 is wound at a carrier roller 33, and moved forward by the rotating movement of the carrier roller 33 and a feed roller 39 between which the recording medium 1 is inserted. When the recording medium 1 reaches a platen 34, the recording operation is established by placing the recording medium against a head cartridge 9 and ejecting ink from the orifices of the recording head. After completing the recording operations, the recording medium 1 is moved by the extraction roller 41 and a spur 42 and finally, extracted from an extraction port 8 of a housing case 7. In FIG. 1, the head cartridge 9 is mounted on a carriage 11 which is moved along a guide shaft 23 extended in the main-scan direction.
In the prior art ink jet recording apparatus, at the state shown in FIG. 1 in which the edge of the recording medium 1 comes near the extraction roller 41, the recording medium 1 is placed on the line defined by the common tangent line between the carrier roller 33 and the extraction roller 41 and the recording medium 1 faces against the recording head 9. However, as the recording operations proceed, the edge of the recording medium 1 bends behind the normal position, and as a result, the portion of the recording medium 1 at the platen 34 is apart from the platen 34 and moves closer to the recording head.
FIG. 2 shows the state in which the recording medium 1 bends behind its normal position. In FIG. 2, the portion of the recording medium 1 in the neighboring area of the recording head is displaced left from its ordinary position relative to the recording head, the extraction roller 41 and the feed roller 39. That is, the distance between the surface of the recording head and the uppermost orifice of the recording head and the distance between the surface of the recording head and the lowermost orifice of the recording head are not identical to each other.
The bad effect of the change of the distance between the surface of the recording medium 1 and the recording head over the recording performance is described by referring to FIG. 3. In FIG. 3, Vi is the velocity of ink ejected from the orifice, Vc is the velocity of the carriage moving in the main-scan direction, and Vr is a combination vector of Vi and Vc, and .theta.r is the angle defined between Vc and Vr. ##EQU1##
In the above mentioned ink jet recording apparatus, ##EQU2## wherein "cps" means character/sec, "48 dots" corresponds to the width per character and the dot number per inch is 360.
In addition, the gap between the recording head 9 and the recording medium 1 is determined to be 1 mm. In case that the portion of the recording medium facing to the uppermost orifice of the recording head is not apart from the platen and that the gap between the portion of the recording medium facing to the lowermost orifice (or sixty fourth orifice) of the recording head and the platen (floating value) is supposed to be 0.4 mm, the recording position displacement dx is estimated as in, ##EQU3## And furthermore, in addition to the above condition, suppose that the angle defined between the orifice forming face and the surface of the recording medium is not zero, that is, the angle .theta.. The angle .theta. is defined as shown in FIG. 4. In the previous case, the angle .theta. is defined to be zero. In similar way, the mathematical and geometrical relationship between Vi, Vc, Vr and .theta.r establishes that ##EQU4##
In this example, the angle .theta. is defined to be 100 degrees, supposing that the portion of the recording medium facing to the orifices located at the upper part of the recording head is not apart from the platen and that the distance between the portion of the recording medium facing to the 64th orifice located at the lower part of the recording head and the surface of the platen (floating value) is supposed to be 0.4 mm, the recording position displacement dx is estimated as in ##EQU5##
And thus, what is concluded is that the larger the angle between the orifice forming face and the surface of the recording medium, the larger the effect of the gap between the surface of the platen and the surface of the recording medium over the recording position displacement.
This effect is extremely emerged in case that the recording medium is naturally curly, or that the strong but flexible material is used for the recording medium. In addition, another problem occurs as that the longer the length of the edge of the recording medium in the sub-scan direction, the larger this effect.
In addition, processing precision and fabricating error of hardware components of the recording apparatus may affect the occurrence of the recording position displacement which means that the expected relative position between the recording head and the recording medium is not established in the actually assembled apparatus.
By ejecting ink on the recording medium, the portion of the recording medium absorbing the projected ink expands. In such a case, as flat portions and expanded portions are distributed randomly on the recording medium in accordance with the recording operation for forming recording images, convex parts and concave parts are formed on the recording medium. So far, not only the convex and concave parts are distributed in the sub-scan direction along which the recording medium is fed forward, but also convex and concave parts are distributed in the man-scan direction along which the carriage having the recording head moves vertically facing against the recording medium.
Thus, the height of convex and concave parts is subject to the quantity of ink projected on the recording medium; the overall quantity of ink projected on the recording medium for forming visual images and for color printings using multiple 3 to 4 tones of colored ink is larger than that for recording character fonts, and hence, as the height of convex parts is larger in such cases, satisfactory level for establishing qualified recording images can not be obtained in spite of requesting high quality recording images for visual images and multiple color printings.
In general, in the ink jet recording apparatus, the diameter of the ink dot projected on the surface of the recording medium is subject to several factors; the kind of recording media, such as used materials, sizes and thicknesses, and surface finish professing; the characteristics of individual recording heads; operational environments of the recording apparatus; the history of recording operations; and the amount of ink remained in the ink storage, expected level of recording quality can not be obtained. In few special cases but in the worst case, the ink can not be ejected from the orifice, and the high-quality recording images can not be obtained. In general, these problems are recognized to be improved.
From another view of the improvement of the overall recording apparatus in its operational environment, there is a problem in the recording position displacement between the images recorded on the previous lines and the images to be recorded on the current line.
In the prior art recording apparatus as described above, even in case that the ink can not be ejected from the recording head, the drive signal for requesting the recording head to eject ink is still continuously supplied to the designated electro-thermal conversion devices at the ejection heater part of the recording head. If this supply of drive signals to the ejection heater part of the recording head continues too long, thermal energy to be removed in accordance with the ejection of ink is stored at the ejection heater part, which may cause a bad effect over its neighboring orifices operated under normal conditions and even cause the disconnection of wires for transmission of drive signals to the recording heads, and thus, these problems related to the performance of the recording head itself should be solved.
And furthermore, even if the drive signal for requesting the recording head to eject ink is disconnected only when the drive signal is directed to the orifice in which what is observed is that the ink can not be ejected, the recorded image may contain white or blank spots corresponding to orifices from which ink can not be ejected, and therefore, what is remained as a problem is that designated recorded images can not be obtained.